The objective of this investigation is to establish the degree of heterogeneity of human lymphocyte population based on the binding properties of a panel of bacterial strains to establish differences in this respect between normal individuals and patients with leukemia and other diseases with lymphoid cell abnormalities. Our preliminary results based on screening 53 different strains of bacteria showed that some bacterial strains did not bind to lymphocytes, some were able to bind to B cells, some to B cells and to subsets of T cells and others were able to bind T cell subpopulations. Five subpopulations of human lymphocytes were distinguished based on the binding of bacteria. Three bacterial strains were found which recognized the normal B cell population of blood lymphocytes. One of these strains recognized normal B cells as well as leukemic B cells. The same individual had similar percentages of lymphocytes binding bacteria at different time intervals and different individuals had similar percentages suggesting that these lymphocyte populations can be considered as biological constants. The patterns of binding bacteria by lymphocyte populations from eight leukemic patients showed that some bacteria were able to distinguish between leukemic and normal lymphocyte populations wile others could differentiate among leukemias. Four patterns appeared as a minimum number among the leukemia patients examined thus far. The rationale of this investigation is that the high diversity of bacteria offers the possibility to investigate the heterogeneity of the human lymphocyte population and its changes during malignant proliferation or other diseases with lymphocyte involvement. In summary, our aims are: a) to obtain a "mapping" of human lymphocyte subpopulations and their changes in disease; b) to investigate the mechanisms of binding of bacteria to normal and leukemic lymphocytes; c) to separate and characterize functionally the lymphocyte subpopulations. The binding of bacteria to lymphocytes can be studied by direct observations of blood smears which allows the direct clinical application of the data obtained in this study.